Tweets are 140 characters long. There are 26 letters in English-27 if you include spaces. Using that alphabet, there are 27140≈10200 possible strings. But Twitter doesn't limit you to those characters, though. You have all of Unicode to play with, which has room for over a million different characters. The way Twitter counts Unicode characters is complicated, but the number of possible strings could be as high as 10800.

But that would create a jumble of horrible characters that don't make any sense. So what about combinations of those characters that, say, make sense to English speakers? Turns out, that's a very difficult question to answer:

Claude Shannon... had a clever method for measuring the information content of a language. [He] determined that the information content of typical written English was around 1.0 to 1.2 bits per letter. This means that a good compression algorithm should be able to compress ASCII English text—which is eight bits per letter—to about 1/8th of its original size. Indeed, if you use a good file compressor on a .txt ebook, that's about what you'll find.

If a piece of text contains n bits of information, in a sense it means that there are 2n different messages it can convey. There's a bit of mathematical juggling here, but the bottom line is that it suggests there are on the order of about 2140×1.1≈2×1046 meaningfully different English tweets, rather than 10200 or 10800.

That is a staggering number, which would take take a person nearly 1047 seconds to read out loud. And if you want to get a grasp of how long that really is, read Munroe's post. I guarantee you'll be shocked. [What If?]